When the seller says people has called but nobody has an appointment, I insist I WANT THE FIRST CHANCE. And if for our timetable it takes a few days to finally meet and some other buyer arrange to have a more flexible schedule, I insist I want to have the right to decide even if somebody sees it first. In that case I ask the seller to tell the buyer he has promised to me I will have the first option, and in case I don't want it he will be second.
That's a lot of insisting for someone who hasn't actually forked over any cash. When I'm selling something, I Just say that the first person to show up with the correct amount of cash leaves with the merchandise. No matter who called first.
It doesn't work this way in Spain. Vehicle transactions are generally done in two steps: In the first one seller and buyer meet in front of the vehicle and reach an agreement. Wether there is a down payment or not is up to them. The exchange of vehicle and money is generally made in an office where the paperwork is signed at the same time; the office clerk will do the paperwork for the DMV for a fee, but legally, once the clerk witness both parties doing the transaction, the vehicle belongs to the buyer for fines, insurance etc. even when the DMV has not received the title transaction yet.
For that reason, the fact that somebody go to the seller's place and say he wants the vehicle means nothing, not even if he pays an amount as a reserve. Legally, if the buyer backs off the seller has the right to keep the reserve, and if the contrary applies, the seller has to give double amount to the buyer. Problem is that once the money is on the seller's hand how would you force him to pay you double? For that reason, buyers try not to give any money down, and sellers try to avoid putting down prospective buyers even when there has been somebody saying he will buy. Sellers generally say: "somebody says that will buy it, but if you are interested, come over and see it". You just go there and stay in line, if the first buyer backs off you are next.
My brother just happened to buy a car that way. He called the guy and got an answer saying it was sold. My brother told him "in case the buyer backs off please call me". And so it happened. Turned out that the buyer was pissed off saying that it was the seller the one who had to pay the clerk for the paperwork...
You can't expect people here to go and see a vehicle with the cash in hand. You never know wether the ad is a fake and you will find a mofo stealing your money. For big amounts (+4000 euro) generally you prefer a bank transfer because there is always a way to trace the money and a proof of payment.